Direct-conversion photon-counting detectors (PCDs) (for example, CdZnTe or CdTe) suffer from polarization (space-charge build-up in the semiconductor sensor) and electronics pile-up at high X-ray flux levels typically used in CT imaging. A detector-side collimator with small slit-shape openings is used to mitigate this problem. The detector-side collimator has a fixed slit opening size, which is optimized for a certain flux level.
However, the X-ray flux levels in typical CT imaging vary dramatically within a scan. Such a flux level variation depends upon the patient size, shape, and anatomy, as well as the wedge profile. A fixed-size detector-side collimator may be either inadequate in preventing polarization under high flux (opening too large) or insensitive to low-flux conditions (opening too small).